Geography discussion 2

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Geograph110_7_HeatEngine_OceanCirculationI4.pdf

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Climate change

The knobs that control earth’s climate: • Atmospheric composition (greenhouse effect) • Amount of solar radiation (luminosity) • What parts of Earth get radiation (orbit) • Atmospheric and ocean circulation • Earth’s albedo (fraction of solar energy reflected off earth’s

surface) • Volcanoes • Plate tectonics

Let’s learn about the fourth and very important climate knob: Atmospheric and Ocean circulation.

Modern Insolation

As we learned in week 3, the amount of energy received from the sun per unit area varies with la:tude because of the curvature of the Earth’s surface. Modern insola:on is also affected by the shape of the Earth. This figure shows varia:on of incoming solar energy with la:tude. The energy from the Sun radiates outward in all direc:ons; however, by the :me the Sun’s rays reach Earth, they are essen:ally parallel to each other. This means that the flux of solar energy passing perpendicularly through the plane A-B on the right hand side of the figure will be the same at any point. For example, the three “beams” in the diagram are equal in solar flux when they pass through the plane A-B. Because of the curvature of Earth, however, when these beams reach the top of Earth’s atmosphere, the same amount of light is spread over a much larger area at the poles than the equator. Consequently, each unit area of surface receives propor:onately less energy at the higher la:tudes, and the incoming solar flux thus decreases from the equator toward the poles.

Zonal Radiation Balance

The solar radiation absorbed at the surface of the Earth follows the same general pattern as described in the previous slide, although the actual amount absorbed varies with cloud cover and atmospheric absorption. This equator-to-pole gradient in the energy absorbed at the surface exerts a primary control on Earth’s climate. The energy moves from a higher to lower (warm to cold) energy status. So the basic energy on Earth’s surface is shown in the left figure. The right figure shows this incoming energy gradient (orange solid curve) as a function of latitude (i.e. the amount averaged around each latitude band). As you might expect, the maximum absorbed solar energy is found in the tropics, and the available solar energy decreases rapidly as we move toward the poles. This gradient in absorbed solar energy is the single most important control on temperature! More energy is generally available at the equator than at the poles, so we can assume that temperatures should be highest in the tropics and lowest at high latitudes. The same figure also shows the latitudinal distribution of infrared radiation emitted from Earth to space (gray solid curve). The higher emissions in the tropics are a result of the high surface temperatures there. Please note that in the tropics, there is more incoming radiation than actual emission (blackbody radiation). In higher latitudes, there is more back radiation (gray solid curve) than incoming radiation.

The difference between the incoming solar radiation and the outgoing terrestrial radiation is referred to as net radiation. In the right figure, note that the energy absorbed exceeds the energy emitted in the tropics (net radiation is positive); near the poles, the reverse is true (net radiation is negative). This distribution of available energy is a permanent feature of Earth’s climate system. The excess amount of energy is effectively transferred through air (wind) and water (ocean current). (continue)

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So, here is a fact: the energy is transferred from high (warm) to low (cool). Think of this as your cup of hot coffee becoming as cold as room temperature. The pole-to- equator gradient shown in both right and left figures seem to imply that the tropics should get cooler while the poles get progressively warmer. But clearly, this does not happen. Other processes must be operating to ensure an energy balance at each latitude!

Further reading: http://www.physicalgeography.net/fundamentals/7j.html

Convergence, divergence, and the Hadley circulation in the tropics

So, what is really happening in the atmosphere? The figure shows what is called “Hadley circulation” – vertical and horizontal air circulation within the troposphere.

Let’s begin with the heating in the tropics. The large solar input to the tropics heats the surface, which in turn heats the overlying air. When heated from below, air will rise by convection. The tropical air near the surface rises, creating a low-pressure region there. But we know from our everyday weather forecasts that air tends to move horizontally from an area of higher pressure to an area of lower pressure (this is known as pressure gradient force: PGF). Thus, the rising air is replaced by surface air moving equatorward into the region of low pressure from regions of higher pressure. The merging of air masses that are moving inward toward a low-pressure region is called convergence. The converging air masses that meet at the tropics and rise make up the intertropical convergence zone (ITCZ). The surface heating produces evaporation in addition to convection. As the convection air rises, it cools, and the evaporated water (water vapor) in the convecting column condenses to form clouds. As a consequence, the ITCZ is characterized by extensive areas of cloud cover and heavy precipitation.

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We can see the ITCZ from space – thick cloud coverage near the equator exists due to the convergence of warm moist air and the formation of cloud!

Convergence, divergence, and the Hadley circula4on in the tropics

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The top of the troposphere, located at about 12-15 km in the tropics, forms a barrier to further uplift (because, unlike within the troposphere, temperatures generally increase with height in the stratosphere, which prevents convection of air from below). The air that rises in the ITCZ, upon reaching this barrier, is forced to diverge poleward. Divergence, in this case, refers to the movement of air outward from a region in the atmosphere.

This poleward-moving air cools and subsides at about 30N and 30S latitude, creating a high-pressure region and replacing the air that is moving equator-ward at the surface. The air warms as it sinks, which prevents condensation from occurring and clouds from forming. As a result, these regions (of divergence) are characterized by clear skies and low rainfall amounts.

This pattern of air movement, with convergence occurring in the tropics and divergence and subsidence occurring some 30 degrees away in one large convection cell, is called Hadley circulation. This circulation pattern was named for George Hadley, the British meteorologist who first explained this phenomenon. The convection cells on either side of the equator, referred to as Hadley cells, represent the dominant north-south mode of circulation between 30N and 30S latitude.

Convergence, divergence, and the Hadley circulation in the tropics

Please note that the Hadley cells – and the ITCZ – are not continuous around the globe. The circulation takes place in individual cells of rising and subsiding air, and the pattern is further broken up by land-ocean contrasts. The ITCZ is most obvious in the Atlantic and Pacific oceans and is readily observed in satellite images. The large-scale circulation, on the other hand, in Southeast Asia and the Indian Ocean is dominated by the monsoon, and will be described later this semester.

If you check an atlas, you will find that the areas of divergence coincide with some of the world’s largest deserts (e.g., the Sahara and Arabian deserts and the Great Australian Desert). A line of convective clouds marks the ITCZ just north of the equator. The clear areas to the north and south of the ITCZ mark the descending arms of the Hadley cells!

This is a 2-D view of the wind pattern shown in previous slides. There are broken up pieces of cells approximately at equator, 30N and 30S, and 60N and 60S, where surface winds move in opposite directions (due to Hadley circulation). Here is a possible model of the surface wind patterns on a globe. Surface winds blow out of the high-pressure zones at the poles and at 30N and 30S and blow toward the low-pressure zones at the equator and in the mid-latitudes.

But as we all know, this is not a representative pattern of the predominant wind (called prevailing wind). The actual pattern is more complicated as you see in the following slide…

Global Winds

Westerlies

90°N (North Pole)

90°S (South Pole)

60°N

30°N

0° (Equator)

30°S

60°S

Polar Easterlies

Polar Easterlies Polar Front

Polar Front

Trade Winds NE Trade Winds

Trade Winds SE Trade Winds

subtropical high "horse latitudes"

rising air masses

sinking air masses

In reality, surface winds tend to blow in east-west directions as well. Indeed, the east-west motions are considerably greater than the north-south motions.

Why?

These strong east-west movements are caused by….

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Global Wind

• There must be a another force acting on the atmosphere.

It�s called the Coriolis Effect

• The Pressure Gradient Force (PGF) and the Coriolis Effect work together to make the winds blow

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… Coriolis Effect (Force)! So, the importance for global wind is to understand; 1) the pressure gradient force, which initiates the wind blowing, and 2) the Coriolis Effect, which impacts the direction of the wind.

merry-go-round

The person on the outside (#1) travels

faster than the person on the inside (#2)

How does Earth�s rotation cause the

Coriolis Effect?

East-west movements of surface winds are the result of the Coriolis effect. The Coriolis effect – named for Gaspard Gustav de Coriolis, the French mathematician who in 1835 proposed that the concept applies to surface winds – is the apparent tendency for a fluid (air or water) moving across Earth’s surface to be deflected from its straight-line path.

Coriolis Force, in relation to its effect, is only an apparent force due to the observer’s frame of reference, not a real force due to an identifiable source, such as the gravitational pull of a planet.

Viewed from the space, a north-south moving object appears to be deflected to

the east or west, because, just like riding on a marry-go-round, an object in the

equator travels the fastest (approximately 464 m/sec) and it slows down as we

move to the North (or South) Pole. Viewed from space, the same object is in

fact seen move in a straight line. The apparent curve that we see is the result of

our frame of reference – we normally view the object’s movement from within

the system.

The Coriolis effect applies to any object moving on a rotating body!

Suggested YouTube video:

https://youtu.be/aeY9tY9vKgs

Two hours later the Earth has rotated

through 30° of arc

30°W

60°W90°W120°W150°W180°W(180°E)

150°E

0 km/hr @ 90°

800 km/hr @ 60° (497 mi/hr)

1400 km/hr @ 30° (869 mi/hr)

1600 km/hr @ 0° (994 mi/hr)

initial directions (stippled arrows)

actual directions (black arrows)

clear arrows = distance Earth's surface rotated

in two hoursSouthern Hemisphere

Northern Hemisphere

equatorward motion, less deflection

West East

poleward motion; more deflection

Earth�s Rotation and the Coriolis Effect

The Coriolis effect is caused by the different veloci5es on the surface of the Earth at different la5tudes (just like a marry-go-round in previous slide). As a result, there is an apparent deflec5on of air masses, ocean currents and any object moving above the surface of the Earth.

Coriolis Deflection

Objects moving

towards the Poles

Importantly, due to the Earth’s rotation, objects deflect to the right in the Northern Hemisphere, while objects deflect to the left in the Southern Hemisphere.

Two Forces Acting on the Atmosphere: PGF and Coriolis

As a summary, there are three important points on the Coriolis effect:

1) the Coriolis effect is caused by the Earth’s rotation; 2) large air masses and water masses are deflected to the right of the

direction of travel in the Northern Hemisphere and to the left in the Southern Hemisphere, and

3) there is a greater deflection towards the higher latitudes and no effect at the equator.

Global Winds

Westerlies

90°N (North Pole)

90°S (South Pole)

60°N

30°N

0° (Equator)

30°S

60°S

Polar Easterlies

Polar Easterlies Polar Front

Polar Front

Trade Winds NE Trade Winds

Trade Winds SE Trade Winds

subtropical high "horse latitudes"

rising air masses

sinking air masses

This is the figure shown earlier. It shows the heat energy that the tropical ocean receives is transferred to the atmosphere at the equator. This warmed air rises, forming a low-pressure center, and winds blow towards the equator to replace this air. Due to the Coriolis effect, the surface wind is deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. This Hadley cell circulation (and the Coriolis Force) drives the pattern of surface winds across the entire globe. This is called prevailing wind.

Global Winds

Almost the same image to the previous slide, but shown in 3D. It also shows the area of cloud formation at low-pressure region.

L Nor�easter over

the Northeast (3/31/97)

Storms are an important part of seasonal weather. Tropical cyclones (called hurricanes in the Atlantic Ocean and typhoons in the Pacific Ocean) represent safety valves for the release of excess heat that builds up every year in the tropics and subtropics. These powerful seasonal storms transport much of this excess heat towards the cooler high latitudes. Cyclones are driven by the prevailing winds and steered by the Coriolis effect and other low and high pressure cells in their paths as they move to high latitudes.

The figure shows the path of Hurricane Irene in 2011. So, now we know that this hurricane trajectory is influenced by both the prevailing wind and the Coriolis effect (deflected to the right).

Tropical Cyclone in Southern Hemisphere

Tropical Cyclone Evans 2012

Of course, in the southern hemisphere, the Coriolis effect will pull wind to the opposite direc6on (90 degree to the le; as opposed to the right in the northern hemisphere). Therefore, topical cyclone in southern hemisphere rotates clock- wise!